4.7 MULTILEVEL INVERTERS (MLI)

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Main feature

 Ability to reduce the voltage stress on each power device due to the utilization of multiple levels on the DC bus  Important when a high DC side voltage is imposed by an application (e.g. traction systems)  Even at low switching frequencies, smaller distortion in the multilevel inverter AC side waveform can be achieved (with stepped modulation technique

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3 main MLI circuit topologies

NAA-2002 1

MLI (2)

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Diode-clamped multilevel inverter (DCMI)

 Extension of NPC  Based on concept of using diodes to limit power devices voltage stress  Structure and basic operating principle  Consists of series connected capacitors that divide DC bus voltage into a set of capacitor voltages  A DCMI with nl number of levels typically comprises (nl-1) capacitors on the DC bus  Voltage across each capacitor is V DC /(nl-1) ( nl nodes on DC bus, nl levels of output phase voltage , (2nl-1) levels of output line voltage) NAA-2002 2

MLI (3)

V D C V 1 V DC /4 V 2 V V V DC DC DC /4 /4 /4 V 3 V 4 V 5 D c1 D c4 D c3 D c6 D c2 D c5 S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 D 1 D 2 D 3 D 4 D 5 V o D 6 D 7 D 8 NAA-2002 3

MLI (4)

 Output phase voltage can assume any voltage level by selecting any of the nodes  DCMI is considered as a type of multiplexer that attaches the output to one of the available nodes  Consists of main power devices in series with their respective main diodes connected in parallel and clamping diodes  Main diodes conduct only when most upper or lower node is selected  Although main diodes have same voltage rating as main power devices, much lower current rating is allowable  In each phase leg, the forward voltage across each main power device is clamped by the connection of diodes between the main power devices and the nodes NAA-2002 4

MLI (5)

 Number of power devices in ON state for any selection of node is always equal to (nl-1)  Output phase voltage with corresponding switching states of power devices for a 5 level DCMI

Power device Output Phase Voltage (V o ) V 1 V 2 V 3 V 4 V 5 index S 1 1 0 0 0 0 S 2 S 3 1 1 1 1 0 1 0 0 0 0 1 1 1 1 0 S 4 S 5 S 6 S 7 0 0 0 1 0 0 1 1 0 1 1 1 1 1 1 S 8 0 0 0 0 1

NAA-2002 5

MLI (6)

 General features  For three-phase DCMI, the capacitors need to filter only the high-order harmonics of the clamping diodes currents , low-order components intrinsically cancel each other  For DCMI employing step modulation strategy, if nl is sufficiently high, filters may not be required at all due to the significantly low harmonic content  If each clamping diode has same voltage rating as power devices, for nl-level DCMI, number of clamping diodes/phase = (nl-1) x (nl-2)  Each power device block only a capacitor voltage NAA-2002 6

MLI (7)

 Clamping diodes block reverse voltage (Dc1, Dc2, Dc3 block VDC/4, 2VDC/4 and 3VDC/4 respectively)  Unequal conduction duty of the power devices  DCMI with step modulation strategy have problems stabilizing/balancing capacitor voltages  Average current flowing into corresponding inner nodes not equal to zero over one cycle  Not significant in SVC applications involving pure reactive power transfer NAA-2002 7

MLI (8)

 Overcoming capacitor voltage balancing problem  Line-to-line voltage redundancies (phase voltage redundancies not available due to structure)  Carefully designed modulation strategies  Replace capacitors with controlled constant DC voltage source such as PWM voltage regulators or batteries  Interconnection of two DCMIs back-to back with a DC capacitor link (suitable for specific applications only – UPFC, frequency changer, phase shifter) NAA-2002 8

MLI (9)

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Imbricated cell multilevel inverter

 Capable of solving capacitor voltage unbalance problem and excessive diode count requirement in DCMI  Also known as flying capacitor multilevel inverter (capacitors are arranged to float with respect to earth)  Structure and basic operating principle  Employs separate capacitors precharged to [(nl-1)/(nl-1)xVDC], [(nl-2)/(nl-1)xVDC] …{[nl-(nl-1)]/[nl-1]xVDC}  Size of voltage increment between two capacitors defines size of voltage steps in ICMI output voltage waveform NAA-2002 9

MLI (10)

 nl-level ICMI has nl levels output phase voltage and (2nl-1) levels output line voltage V DC 3V DC /4 V DC /2 V DC /4 S 5 S 6 S 7 S 8 S 1 S 2 S 3 S 4 D 1 D 2 D 3 D 4 V o D 5 D 6 D 7 D 8 10 NAA-2002

MLI (11)

 Output voltage produced by switching the right combinations of power devices to allow adding or subtracting of the capacitor voltages  Constraints : capacitors are never shorted to each other and current continuity to the DC bus capacitor is maintained  5-level ICMI – 16 power devices switching combinations (SWC) . To produce VDC and 0 (1 SWC – all upper devices ON, all lower devices ON), VDC/2 (6 SWC), VDC/4 and 3VDC/4 (4 SWC)  Example - capacitor voltage combinations that produce an output phase voltage level of VDC/2 NAA-2002 11

MLI (12)

VDC - VDC/2 VDC – 3VDC/4 + VDC/4 VDC - 3VDC/4 +VDC/2 – VDC/4 3VDC/4 – VDC/2 + VDC/4 3VDC/4 – VDC/4 VDC/2  Power devices switching states of a 5-level ICMI Power device Output Phase Voltage (V o ) V 1 V 2 V 3 V 4 V 5 index 1 0 0 0 0 S 1 S 2 S 3 S 4 S 5 1 1 1 0 1 1 1 1 0 1 1 1 0 0 1 1 0 0 0 1 S 6 S 7 S 8 0 0 0 0 0 0 1 0 0 1 1 0 1 1 1 NAA-2002 12

MLI (13)

 General features  With step modulation strategy, with sufficiently high nl, harmonic content can be low enough to avoid the need for filters  Advantage of inner voltage levels redundancies - allows preferential charging or discharging of individual capacitors, facilitates manipulation of capacitor voltages so that their proper values are maintained  Active and reactive power flow can be controlled (complex selection of power devices combination,  switching frequency/losses for the former)  Additional circuit required for initial charging of capacitors NAA-2002 13

MLI (14)

 Assuming each capacitor used has the same voltage rating as the power devices, nl-level ICMI requires: (nl – 1) x (nl – 2)/2 auxiliary capacitors per phase (nl – 1) main DC bus capacitors  Unequal conduction duty of power devices 

Modular structured multilevel inverter (MSMI)

 Referred to as cascaded-inverters with Separate DC Sources (SDCs) or series connected H-bridge inverters  Structure and basic operating principle NAA-2002 14

MLI (15)

 Consists of (nl–1)/2 or h number of single phase H-bridge inverters (MSMI modules)  MSMI output phase voltage Vo = Vm1 + Vm2 + …….. Vmh Vm1 : output voltage of module 1 Vm2 : output voltage of module 2 Vmh : output voltage of module h • Structure of a single-phase nl-level MSMI NAA-2002 15

V DC V DC V DC

MLI (16)

S 11 S 21 V m1 Vphase (V o ) S 31 Module 1 S 12 S 41 S 22 S 32 Module 2 S 42 V m2 S 1h S 2h S 3h Module h S 4h V mh 0 NAA-2002 16

MLI (17)

 Power devices switching states of a 5-level MSMI Power devices index Output voltages S 11 S 21 S 31 S 41 S 12 S 22 S 32 S 42 1 0 0 1 1 0 0 1 1 1 1 0 0 0 0 0 0 1 1 1 1 0 0 1 0 1 0 1 1 0 0 0 V m1 V m2 V o +V DC +V DC +2V DC +V DC 0 +V DC +V DC +V DC 0  V DC +V DC 0 1 1 1 1 0 0 0 0 1 1 0 1 0 0 1 0 0 0 +V 0 DC +V 0 DC 1 1 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 1 1 0 1 1 1 1 1 1 1 1 0 1 1 1 1 0 0 0 0 0 1 1 0 0 1 1 0 0 1 0 1 0 1 0 1 0 1 1 0 0 1 1 0 0 1 1 0 1 0 0 0 1 0 0 0 0 0 0 0  V DC +V DC 0 0  V DC +V DC 0 0 0 0 0 0  V DC  V DC +V DC  V DC  V DC  V DC  0 0 V DC    V 0 V V -2V DC DC DC DC NAA-2002 17

MLI (18)

 General features  Known to eliminate the excessively large number of bulky transformers required by the multipulse inverters, clamping diodes required by the DCMIs and capacitors required by the ICMIs  Simple and modular configuration  Requires least number of components  Comparison of power devices requirements per phase leg among three MLI (assuming all power devices have same voltage rating, not necessary same current rating, each MSMI module represented by a full-bridge, DCMI and ICMI use half-bridge topology) NAA-2002 18

MLI (19)

Type of multilevel inverter Main power devices Main diodes Clamping diodes DCMI (nl – 1) x 2 (nl – 1) x 2 (nl - 1) x (nl - 2) DC bus capacitors Balancing capacitors (nl – 1) 0 ICMI (nl – 1) x 2 (nl – 1) x 2 0 (nl – 1) (nl – 1) x (nl – 2)/2 MSMI (nl – 1) x 2 (nl – 1) x 2 0 (nl – 1)/2 0  Flexibility in extending to higher number of levels without undue increase in circuit complexity simplifies fault finding and repair, facilitates packaging  Requires DC sources isolated from one another for each module for applications involving real power transfer  Adaptation measures have to be taken in complying to the separate DC sources requirement for ASDs applications NAA-2002 19

MLI (20)

– Feed each MSMI module from a capacitively smooth fully controlled three phase rectifier, isolation achieved using specially designed transformer having separate secondary windings/module – Employ a DC-DC converter with medium to high frequency transformers (between rectifier output and each MSMI module input), allows bidirectional power flow  Isolated DC sources not required for applications involving pure reactive power transfer (SVG)  pure reactive power drawn, phase voltage and current 90º apart  balanced capacitor charge and discharge NAA-2002 20

MLI (21)

 Originally isolated DC voltages, alternate sources of energy (PV arrays, fuel cells)  Advantage of availability of output phase voltage redundancies  Allows optimised cyclic use of power devices to ensure symmetrical utilization, symmetrical thermal problems and wear  Design of power devices utilization pattern possible  Overall improvement in MSMI performance – high quality output voltage etc.

NAA-2002 21

MLI (22)

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Modulation strategies for multilevel inverters

 Step modulation  Space vector modulation  Optimal/programmed PWM technique  Sigma delta modulation (SDM)  High-dynamic control strategies   Multilevel hysterisis modulation strategy Sliding mode control based on theory of Variable Structure Control System (VSCS) NAA-2002 22